Thermal Switch

ABSTRACT

The invention concerns a thermal switch, characterized in that said switch comprises a thermal connection element consisting of an integral single part forming a body of thermally conductive homogeneous composition as well as an actuator for opening said switch by mechanical rupture of said thermal connection element.

This invention relates to a thermal switch.

The invention applies in very general terms to any situation in whichtwo mechanically and thermally coupled components must be thermallydecoupled.

In this context, the invention is of particularly advantageousapplication to the cryogenic cooling of space equipment.

At the present time, two types of thermal switches are known:

-   -   differentially expanding thermal switches based on the principle        of the change in the length of materials in response to the        temperature of the system. Below a certain temperature,        contraction of one of the materials brings the different parts        into contact, thus providing the thermal connection. Above this        temperature, however, the contact between the different parts is        broken, along with thermal connection,    -   phase-change thermal switches based on the change of state of a        fluid, the fluid being chosen to suit the operating temperature        of the system. If it is a gaseous fluid, it will condense onto        the cold part of the system and evaporate on the hot part.        Transfer of mass is effected by gravity in a ground-based        application and by capillarity in a micro-gravity application.        To thermally disconnect the components, the system is purged of        its gas, or alternatively the gas is transferred to a similar        system in the case of redundancy.

Document U.S. Pat. No. 5,522,226 describes a thermal transfer deviceconsisting of two separate components connected by means of anintermediate layer of iridium.

Document U.S. Pat. No. 4,384,610 describes a thermal coupling systemusing a member consisting of two different components separated orconnected via a metal layer which can be melted by a temperaturecontrol.

The main problems with phase-change thermal switches are:

-   -   the dependency on the operating temperature determined by the        materials,    -   the large thermal resistance of the connection,    -   a contact resistance which is large and difficult to control,    -   the need for long components in order to achieve sufficient        movement to thermally connect or disconnect the components,    -   significant mechanical constraints in terms of manufacture due        to small clearances between components,    -   the risk of the components bonding to the point where they can        no longer be separated.

The main problems with phase-change thermal switches are:

-   -   the dependency on the operating temperature determined by the        fluid used,    -   the limited number of fluids in existence, which limits the        ranges of use,    -   the problems of pressure containment and sealing,    -   pollution-related problems during evacuation of the fluid from        the system,    -   large dimensions due to the pumping ports, safety components and        transfer tubes,    -   the difficulty of controlling phase changes in microgravity        situations,    -   the fact that the service life and the performance of the        connection are limited by the leakage rate of the system,    -   the fact that the thermal resistance of the connection in the        non-conducting state, without fluid, is determined by sizing for        mechanical strength associated with pressure.

It is therefore an object of the invention to solve all or some of theproblems encountered with existing thermal switches, especially thoseassociated with the limited range of operation, size and complexity ofimplementation.

The thermal connection is thus made by a material having very goodthermal conductivity. To thermally disconnect the system, an externalaction mechanically ruptures this connection.

It is therefore a two-state irreversible device which offers theadvantage of providing an excellent thermal link in nominal operation,yet being an excellent heat insulator once the mechanical connection isbroken.

The other advantages of the thermal switch in accordance with theinvention are:

-   -   nominal operation is completely passive. No phase change and no        moving parts are required to make the initial thermal        connection. The connection is naturally heat-conducting. When        disconnection becomes necessary, all that is required is to        activate the actuator in order to exert the necessary force to        rupture the thermal connection element. Once the connection is        broken, the force applied by the actuator can be removed and the        thermal connection element is thermally insulating and once        again passive,    -   it operates independently of the temperature of the system.        Specifically, unlike phase-change or differential-expansion        thermal switches which rely on the temperature of the change of        state of a fluid or on the expansion of a material, the thermal        switch of the invention is independent of the operating        temperature and can be used over a wide range of temperatures,    -   it is very small, unlike phase-change thermal switches in        particular, which require filling ports and safety members. The        thermal switch of the invention is compact and low-weight.

Advantageously, said thermal connection element is mechanicallyresilient, which means that the forces that have to be applied by theactuator to said thermal connection element can be kept low.

In order that the force of the actuator can be removed, the inventionprovides that it is suitable for being kept open by return meansfollowing mechanical rupture of the thermal connection element.

In a first embodiment of the invention, said return means are externalto said switch.

In this case, it is provided that said switch comprises aheat-insulating connection member, in order to maintain the mechanicalcohesion of its various constituent components.

To limit the range of movement of the components after the thermalconnection element has been ruptured, said connection member comprises amechanical stop.

In a second embodiment of the invention, said return means consist of anelastic member connected to said switch. This member may be aprestressed bellows fitted around the switch.

The description which now follows with reference to the appendeddrawings, offered by way of non-restrictive examples, will show clearlythe nature of the invention and the manner in which it can beimplemented.

FIG. 1 a is a diagram of a first embodiment of a thermal switchaccording to the invention in the conducting state.

FIG. 1 b is a diagram of the thermal switch as shown in FIG. 1 a in theheat-insulating state.

FIG. 2 a is a diagram of a second embodiment of a thermal switchaccording to the invention in the conducting state.

FIG. 2 b is a diagram of the thermal switch as shown in FIG. 2 a in theheat-insulating state.

FIG. 3 is a diagram showing an example of an application of a switch inaccordance with the invention.

FIGS. 1 a and 1 b show a thermal switch designed to provide thermalcontact or, on the other hand, thermal insulation, between twocomponents arranged one on interface 1 and one on interface 2 of theswitch. As will be seen later in detail, these components may be, forexample, a heat sink and a piece of equipment that must be cooled.

As FIGS. 1 a and 1 b show, the thermal switch comprises a thermalconnection element 10 between the two interfaces 1, 2.

In the nominal conducting state of the switch shown in FIG. 1 a, theelement 10 has sufficient thermal conductivity to exhibit a shallowthermal gradient between the interfaces 1, 2.

If it is necessary to thermally decouple the interfaces 1, 2, anactuator 20 is activated to apply a force to the thermal connectionelement 10 sufficient to mechanically rupture said element 10. In orderto reduce the force and the size of the actuator 20 and control thebreak, the thermal connection element 10 is made resilient by means of,for example, a frangible zone 11 in the form of a waist pre-formed intothe element 10.

FIG. 1 b shows the switch from FIG. 1 a in the heat-insulating state,following rupture of the element 10.

To maintain an effective distance between the two remaining pieces ofthe element 10, return means, such as an external spring (not shown) areprovided to exert a return force on the interface 1, for example, andkeep the thermal switch open following mechanical rupture of the element10.

As can be seen in FIGS. 1 a and 1 b, both the stiffness of the switchassembly and the guidance of the components following rupture aremaintained by a mechanically rigid and heat-insulating connecting member30. A mechanical stop 31 can be used to limit the movement of thecomponents after rupture of the thermal connection element 10.

The embodiment shown in FIGS. 2 a and 2 b differs from that describedabove with reference to FIGS. 1 a and 1 b in the return means employed.In this second embodiment, the return force is exerted by an elasticmember 40 connected to the switch. In FIGS. 2 a and 2 b, this member 40is a bellows installed in a prestressed condition.

The advantage of such a switch structure is that it produces acompletely integrated system that is totally isolated from the externalenvironment.

The materials offering the best compromise between mechanical propertiesand thermal properties when producing the thermal connection element 10are:

-   -   corundums (sapphire, ruby, etc.),    -   ceramics (silicon carbide, tungsten carbide, etc.), and    -   diamond, graphite, silicon, quartz, glass and all metals having        a ductile-fragile transition at low temperatures.

These materials have mechanical properties favorable to the operation ofthe switch of the invention and optimal thermal conductivity atcryogenic temperatures.

The crystalline structure of these materials offers the furtheradvantage of low tensile strength and limited elongation at break.

Unlike other thermal connections which rely on the contact between twoor more walls, our thermal switch adds no internal contact resistancebecause of the use of a one-piece homogeneous connection element 10. Thethermal performance of the proposed system is therefore optimal.

The invention thus has the following advantages over the knownsolutions.

The one-piece homogeneous structure of the connection element 10 avoidsthe need for an internal junction within the connection element 10. Thesystem can therefore be subjected to higher mechanical loads, aparticular advantage for a space application.

Also, the one-piece homogeneous structure has better thermalconductivity than elements with one or more internal connections. Theone-piece element does not have the contact resistances found in priorart arrangements, which can be of the same order of magnitude as withthe thermal resistance of the complete system.

Again, the homogeneous one-piece structure greatly reduces the problemsof assembly, fabrication and hence reproducibility of the switch. Theseare often major problems in space applications. The connection element10, once built, requires no other preparation or the use of extramaterials necessary to transport the heat in particular.

Further, the one-piece element consisting of a component of homogeneouscomposition performs the principal function, which is to transport heat,with certainty. The reliability of such a system is much higher thanthat of any other system based on phase changes of materials ordifferential expansion.

To control the rupture, a frangible zone is created in the element 10 inthe form of a reduced cross section in which the maximum stress islocalized.

For reasons of mechanical strength, especially in space applications,the position of the frangible zone can vary along the element 10. Thisposition can be optimized on the basis of the mechanical spectrum.

The actuators employed are preferably piezoelectric actuators. They havethe advantage of being compact and of being able to exert large forces.The actuators are actuated by an electrical voltage, generate theirforce, and then resume their initial state as soon as the electricalpower is cut off.

Piezoelectric actuators are moreover compatible with cryogenic operationin a vacuum.

The rigid connecting member 30 gives the components mechanical guidance.However, after rupture, conduction losses through this member must beminimized to ensure that the thermal disconnection is as efficient aspossible between the two interfaces 1, 2. Materials which may be usedare glass fibers and epoxy resins.

Lastly, the bellows 40 may be made of thin stainless steel. Itsdeveloped length should be as long as possible to reduce conductionlosses.

One application of the thermal switch according to the invention isthermally to connect cryogenic coolers to equipment for spaceapplications requiring redundancy. The equipment may for example bedetectors, filters, amplifiers, bolometers, screens, mirrors, optics,etc.

The problems of redundancy in vehicular applications lead engineers toconnect a single piece of equipment to several cryogenic coolers.

In ordinary cryogenic chains, the equipment to be cooled is connectedthermally to the cold fingers of two duplicate coolers. The thermal linkbetween the equipment and the cold fingers is provided by a flexiblemetal mesh, of copper for example. Should one of the coolers fail, thefailed cooler not only ceases to participate in the cooling of theequipment but also adds an additional thermal load to thestill-functioning cooler.

The thermal switch of the invention can be inserted into theconventional cryogenic chain to disconnect a failed cooler and eliminatethe undesired losses which it produces when it is out of action.

FIG. 3 shows a setup for a redundant cooling system incorporatingthermal switches in accordance with the invention.

The coolers 100, 200 may for example be Stirling, Gifford-McMahon, orpulse tube machines or any other cooler such as Joule-Thomson, Peltier,adsorption or other coolers. The cold fingers 101, 201 of these coolersare connected to the equipment 300 to be cooled either via a conductingmetal mesh 110, 210 and a thermal switch 120, 220 mounted between themesh and the equipment, or directly on the thermal switch.

When operating nominally, the thermal performance of the switches 120,220 is optimal, and so the energy budget of the system is not degradedby the presence of these switches. The two coolers run at 50% load (forexample), and each supply 50% of the requirement. There is therefore nothermal loss coming from the coolers.

In the event of failure of cooler 200, as indicated in FIG. 3, thethermal switch 220 is activated. The still-active cooler 100 runs atfull power and supports the thermal load on its own, while the failedcooler 200 is disconnected thermally by the action of the switch 220.There is therefore no additional load on cooler 100 and the thermalbudget is unaffected.

1-13. (canceled)
 14. A thermal switch comprising a thermal connectionelement that includes a one-piece component forming a heat-conductingbody of homogeneous composition and an actuator capable of opening theswitch by mechanically rupturing the thermal connection element.
 15. Thethermal switch of claim 14, wherein the thermal connection element ismechanically resilient.
 16. The thermal switch of claim 14, wherein thethermal switch is suitable for being kept open by return means followingmechanical rupture of the thermal connection element.
 17. The thermalswitch of claim 16, wherein the return means are external to the thermalswitch.
 18. The thermal switch of claim 17, wherein the external meanscomprises a heat-insulating connecting member.
 19. The thermal switch ofclaim 18, wherein the connecting member comprises a mechanical stop. 20.The thermal switch of claim 16, wherein the return means comprises anelastic member connected to the thermal switch.
 21. The thermal switchof claim 14, wherein the thermal connection element is made of acorundum material.
 22. The thermal switch of claim 21 wherein thecorundum material is selected from sapphire and ruby.
 23. The thermalswitch of claim 14, wherein the thermal connection element is made of aceramic.
 24. The thermal switch of claim 23, wherein the ceramic isselected from silicon carbide and tungsten carbide.
 25. The thermalswitch of claim 14, wherein the thermal connection element is made of amaterial selected from diamond, graphite, silicon, quartz, and glass.26. The thermal switch of claim 14, wherein the actuator is apiezoelectric component.
 27. The thermal switch of claim 14, wherein thethermal switch is utilized in a cooling system.
 28. The thermal switchof claim 14, wherein the thermal switch is utilized in a heating system.29. The thermal switch of claim 14, wherein the thermal switch isutilized in a ground based or on-board cryogenic cooling system.
 30. Thethermal switch of claim 14, wherein the system is with redundancy. 31.The thermal switch of claim 14, wherein the system is withoutredundancy.